1. Field of the Invention
The present invention relates to a digitally controlled rectifying system used for driving an electric motor.
2. Description of the Prior Art
Conventionally, there have been used rectifying systems of the analog control type for driving electric motors as shown in FIG. 1. The system arrangement of FIG. 1 includes a motor driving rectifying system 1, a d.c. motor 2, a forward converter 3 made up of a 3-phase thyristor bridge, and a reverse converter 4 also made up of a 3-phase thyristor bridge. The control system is of a feedback system comprising a speed control loop and two minor loops of the current and voltage feedback.
The rectifying system 1 incorporates a speed controller 5 made of an operational amplifier configured to provide proportional and integral functions, a terminal 6 for receiving a speed command signal, a terminal 7 for receiving a feedback speed signal which is produced uninterruptedly by a pilot generator 8 associated with the motor 2, a current controller 9 made up of an operational amplifier configured to provide an integral function, an output terminal 10 of the speed controller 5 which provides the current command signal, a terminal 11 for receiving a feedback current signal which is produced in response to the input a.c. current detected by an a.c. current transformer (ACCT) 12, a voltage controller 13 made up of an operational amplifier configured to provide a first-order time lag function, an output terminal 14 of the current controller 9 which provides a voltage command signal, a terminal 15 for receiving a feedback voltage signal which is produced in response to the d.c. output voltage by a voltage sensor 16 provided across the motor 2, a gate pulse generator 17 incorporating a cos.sup.-1 function for providing a linearized output voltage for a phase command signal with a bias voltage Eb being applied thereto in order to stabilize the gate switching operation for the forward and reverse converters 3 and 4 as will be explained later, a forward/reverse switching logic circuit 18 for selecting the output of the gate pulse signal (a) and (b) for the forward and reverse converters 3 and 4, and gate pulse switches 28a and 28b for conducting gate pulses to the forward and reverse converters 3 and 4.
The forward/reverse switching operation of the foregoing conventional rectifying system will be described.
Symbols .sym. and .crclbar. shown at the inputs and outputs of the speed, current and voltage controllers 5, 9 and 13 represent the polarity of respective control signals when the motor 2 rotates in a forward direction.
FIG. 2 is a graph used to explain the correlation between the input signal of the gate pulse generator 17, i.e., the output signal 19 of the voltage controller 13, and the d.c. output voltage. In the figure, the solid line shown by A represents the characteristics when the load current flows continuously, the shaded portion shown by B represents the characteristics when the load current flows intermittently, and the dashed lines shown by C represent the state in which the load current is completely cut off. The provision of the dead band is to prevent the forward and reverse converters 3 and 4 from becoming conductive simultaneously, and for this purpose the output characteristics of both converters are biased by a predetermined amount of .+-.Eb volts with respect to the output signal of the voltage controller 13. The voltage level shown by Ec in the graph represents the counter electromotive force produced by the motor 2 when it rotates in the forward direction. When the speed controller 5 is given a deceleration command via the terminal 6, its output signal at the terminal 10 decreases from a positive value and then enters the negative region. Since the current controller 9 has an integral property, its negative output signal at the terminal 14 when evaluated as an absolute value starts decreasing at a rate proportional to the product of the reciprocal of the integrating time constant and the input error signal. On the other hand when the output signal 19 of the voltage controller 13 has decreased down to the input signal level V.sub.1, the load current becomes a complete zero, causing the output signal of the current controller 9 to vary solely in response to the current command signal at the terminal 10.
When the voltage controller 13 has reversed the polarity of the output signal from positive to negative, the switching logic circuit 18 operates on the gate pulse signal by its output signals Sa and Sb to switch from the forward converter 3 to the reverse converter 4. When the output signal 19 has reached the input signal level -V.sub.2, the current starts flowing through the reverse converter 4, bringing the motor 2 in a regenerative operating mode, and the motor speed starts falling. In the meantime when the output signal 19 varies from the input level V.sub.1 to -V.sub.2, no load current flows in the motor 2, and it is an idle time for the control system. This idle time cannot be nullified due to the bias Eb provided for the output of the voltage controller 13 in order to prevent a short-circuit of the forward and reverse converters 3 and 4, and to make the matter worse the idle time is significantly affected by the gain of the current controller 9 and voltage controller 13.
Therefore, it has been logically impossible for the conventional motor driving rectifying system of the analog control type to reduce the switching time because of the need for the bias voltage Eb for stabilizing the switching operation of the forward and reverse converters. Moreover, the gain of the current controller 9, which is determined depending on the properties of the motor and load, could cause a very long idle time for the converter switching operation in some cases.